The present invention relates generally to readout systems for imaging detectors, and more specifically to a readout systems that provides improved resolution and increased field of view for imaging detectors such as PS-PMT""s.
A Position Sensitive Photo-Multiplier Tube (PS-PMT) is an imaging detector that converts light photons into an electrical current. The main components of a PS-PMT are an input window, a photocathode, focusing electrodes, dynodes and at least one anode (output). The photocathode is used for converting incoming light (photons) into electrons. These photoelectrons, which are a product of photoelectric effect, are directed by the potential of focusing electrodes towards dynodes. The dynodes are used to multiply the electrons by the process of secondary electron emission. Electron gains of 103 to 108 are common and depend on the number of dynodes and inter-dynode potentials. Dynodes are made of or covered with a layer of secondary emissive material. The condition of the dynode surfaces are responsible for PS-PMT stable gain performance.
The voltages that create the electrostatic fields between the photocathode, the focusing electrodes and the dynodes are delivered from a single high-voltage stable power supply and a voltage divider. The divider is a common part of a PS-PMT base. The design of the divider circuit is crucial to getting the best performance from the PS-PMT. There are many versions of PS-PMT high voltage dividers optimized or designed for some particular application. Most of them are concentrated on specific parameters that are critical for a given application, such as maximum gain, dynamic range, low noise, or linearity.
Series-regulator type high voltage power supplies optimized for photomuliplier tubes are well known in the art and have gained a good reputation. Other components found in or required by scintillation cameras, PS-PS-PMTs, are described in xe2x80x9cPhotomultiplier Tube, Principle to Applicationxe2x80x9d by Hamamatsu Photonics K. K, March 1994, which is incorporated herein by reference.
The output of a photomuliplier tube is a current (charge), while the external signal processing circuits are usually designed to handle a voltage signal. Therefore, the current output must be converted into a voltage signal by a current to voltage converter. Further, the current that is output from a PS-PMT anode is very small, especially in low light level detection, low gain PS PMT""s, and photon counting applications. An operational amplifier can be used to both convert the anode output current to a voltage and accurately amplify the resulting voltage. Typically this operational amplifier is powered by a source that is separate from the high voltage power source for the dynode stages of the PS-PMT. This is done to insure the stability of the power supply to the dynodes.
PS-PMTs generally have multiple anode outputs that are arranged in arrays with X number of rows and Y number of columns. The analog signal outputs from the anodes can be processed individually or combined in a variety of ways and the results analyzed using appropriate data acquisition systems under computer control. The highest resolution readout systems process each anode output individually. These individual wire readout systems provide high quality display information, however, they also require large numbers of circuits, wires and cables. The physical space requirements of individual wire readouts becomes prohibitive for larger arrays. These systems also produce extremely large amounts of data for the associated software to process. Another traditional readout system uses charge division resistor chains to combine the outputs of the rows and columns. This traditional readout system reduces the number of outputs from each row and each column to two. Such systems reduce the amount of required circuits, wires and cables, and the amount of data to be processed. However, this system also reduces resolution and field of view compared to individual wire readout systems. Processed data from readout systems can then be displayed on a video monitor for further study of the subject being imaged.
What is needed is a readout system that requires a small amount of circuits, wires and cables, like the traditional charge division resistor chain system, but one that also provides high resolution and does not decrease the field of view like the individual wire readout system.
A readout circuit, for an imaging detector, that improves resolution and increases the field of view in a display. The readout circuit comprises a fractional subtraction circuit and rail to rail amplifiers which only allow signals of one polarity to be transmitted to a combining circuit. The combining circuit receives the unipolar signals and combines them to produce two readout circuit outputs. The outputs are then sent to the display.
The readout circuits are advantageously used in an imaging detector that comprises one or more pairs of readout circuits, wherein each pair of readout circuits comprises an X readout circuit and a Y readout circuit. Each X readout circuit receiving a row of outputs from an imaging detector and each Y readout circuit receiving a column of outputs from the imaging detector.
In one embodiment the combining circuit is a resistive charge division circuit. In an alternative embodiment the combining circuit is a weighted summing circuit. The number of outputs received from the imaging detector is typically 200 or less, however the number of outputs from each readout circuit remains constant at two.
It is an object of the present invention to provide a readout system with resolution as good as an individual wire readout system but without requiring as many circuits, wires and cables as individual wire readout systems.
It is another object of the present invention to provide a readout system that provides resolution as good as an individual wire readout system but without generating as much data that must be processed as individual wire readout systems.
It is a further object of the present invention to increase the field of view of a display for a readout circuit that uses a combining circuit such as a charge division resistive chain.